1. Field of the Invention
The present invention relates to an electronic still camera for bracketing a plurality of images having different exposures and, more particularly, to an electronic still camera capable of photographing the plural images at superposed or continuous timings.
2. Description of the Prior Art
In order to obtain an image of optimum exposure, the auto-bracketing may be accomplished for taking a plurality of images of different exposures by a series of operations.
In response to only one depression of a shutter button, for example, the camera itself takes a plurality of shots continuously with varying exposures. If these continuous shots are to be interrupted, the shutter button may be released.
In the case of the auto-bracketing described above, the individual images have slightly different exposure timings. This raises no problem if the object stands still. In the case of a moving object, however, the desired taking timing (i.e., the so-called "shutter chance") is missed even if the shot taken at the instant of depression of the shutter button has an improper exposure and following shot has a proper exposure.
FIGS. 14(A)-14(E) are time charts showing the shutter timings. When the release switch is depressed (or ON, as indicated at 1 in FIG. 14(A)), the iris drive motor for adjusting the iris is energized (as indicated at 2 FIG. 14(C)). After this, the shutter is opened (as indicated at 3 in FIG. 14(B)) to effect the first exposure. This exposure time is slightly shorter than the proper exposure time, for example. At the end of this exposure, there are accomplished the shutter charge (as indicated at 4 in FIG. 14(D)), the film winding (as indicated at 5 in FIG. 14(E)) and the iris drive (as indicated at 6 in FIG. 14(C)). Then, the shutter is opened again (as indicated at 7 in FIG. 14(B)) to effect the second exposure. This exposure time is set to have the proper exposure time, for example. At the end of this exposure, there are accomplished the shutter charge (as indicated at 8 in FIG. 14(D)), the film winding (as indicated at 9 in FIG. 14(E)) and the iris drive (as indicated at .circle. 14 in FIG. 14(C)). Then, the shutter is opened again (as indicated at .circle. 11 in FIG. 14(B)) to effect the third exposure. This exposure time is slightly longer than the proper exposure time, for example. At the end of this exposure, the shutter charge (as indicated at .circle. 12 in FIG. 14(D) and the film winding (as indicated at .circle. 13 in FIG. 14(E)) are accomplished for preparing a subsequent photographing. Thus, three shots of different exposures are executed while the release switch is ON. Incidentally, letters T.sub.1 and T.sub.2 appearing in FIGS. 141A)-14(E) designate the time periods for which the shutter is closed till it is opened.
Here, the camera thus far described is required to have the time periods of T.sub.1 and T.sub.2 of 200 to 300 msecs or more. In the case of the moving object, therefore, the photographing of the proper exposure may possibly come out of the desired taking timing (or the so-called "shutter chance").
In case, moreover, an exposure meter is built in the camera, it is very important problem how the light-receiving element is arranged and what sensitivity distribution (i.e., the sensitivity of the light in each position of the frame and in the luminance distribution pattern) the brightness of the object is to be measured in.
Therefore, there has been used the multi-division photometric system in which the finder frame is divided into a plurality of regions so that it may be measured by the photometric element. FIG. 19 is an explanatory view showing the manner of dividing the frame by the multi-division photometric system. In this example, as shown, the whole frame is divided into nine photometric regions. Of these, the central region is used for the photometry of the main object whereas the surrounding eight regions are used for the photometry of the background.
FIG. 20 presents an example of the finder frame in case a portrait is to be centralized for photographing. In case the object is thus taken against the light, the background receives a larger amount of light whereas the central region or the object receives a smaller amount of light. In this case, the photometric value is calculated (at the central spot) by using not the photometric information of the background but only the photometric information of the center. As stated above, the automatic exposure (AE) is executed while accomplishing the compensation with the backlight.
In addition, the photometry includes the averaged photometry, the central weighted photometry, the high-luminance stressed photometry and the low-luminance stressed photometry for exposure controls, of which the most proper one is selected in accordance with the luminance distribution pattern.
No complete algorithm exists in the exposure controls thus far described. In other words, an algorithm capable of correcting a certain luminance distribution properly might deteriorate another luminance distribution.
In a pattern having an object of lower luminance surrounded by an object of higher luminance, for example, the compensation with the backlight will adjust the exposure to the portion of lower luminance. According to the algorithm of an intense correction with the backlight, however, the exposure is adjusted to the hair, if any in a bright cloth or face, so that the cloth or face will be blanked white. If this blanking were prevented, the correction with the backlight would become ineffective.
In the electronic still camera, generally speaking, the film is replaced by a solid-state image pickup device (which will be called hereinafter as the "CCD") and a magnetic floppy. The CCD photoelectrically converts the light received at its light receiving portion into electric charges and stores the charges in the depletion layer of the substrate of the CCD. On the other hand, a magnetic disc in a magnetic recorder rotates the disc-shaped magnetic floppy to store still images of 50 frames in response to the motions of the head.
In case a film having a narrow latitude such as a positive film is used in a silver halide camera, a plurality of shots are frequently taken for one object under different exposure conditions so as to produce an accurately exposed photograph. This is called the "bracketing." In order to simplify this bracketing, some recent camera is enabled to accomplish the changes in the exposure conditions, the film feed and the photography automatically if the change in the exposure and the number of the shots are set. This technology is called the "auto-bracketing". The bracketing is effective for the electronic still camera because its solid-state image pickup device has a narrow latitude.
In this electronic still camera of the prior art, however, the auto-bracketing requires the use of tracks of the magnetic floppy in the number of bracketed shots.
In the automatic exposure control (which will be shortly referred to as the "AE") of the existing video camera, on the other hand, the extent of iris and the exposure time are so controlled that the values obtained by integrating and averaging the luminance signal levels of the whole image pickup frame detected by the image pickup device may take a proper level. In the other way, the central portion of the frame is weighted, and the luminance signal levels are integrated to control the extent of iris and the exposure time.
In another technology, the frame is divided into a plurality of regions, for which the luminance signal levels are respectively integrated and are subjected to comparisons and weighted additions (including the corrections with the backlight) to control the extent of iris and the exposure time. In the silver halide film type camera, similar operations are accomplished by using the signal outputs of a photometric element provided.
There are other technologies, in which abnormal levels in the vicinity of the saturation level or black level are detected from the luminance signal levels so that the exposure conditions for the lowest abnormal level may be set from a plurality of shots picked up (as disclosed in Japanese Patent Laid-Open No. 60-178779), and in which two shots are taken with the exposures adjusted to set the white and black levels to the grey levels so that the individual photographic signals are synthesized and reproduced (as disclosed in Japanese Patent Laid-Open No. 60-246186).
In the existing system using a value obtained by integrating and averaging the luminance signal levels of the whole image pickup frame, however, the integrated value is increased by the influences of the portion, if any, which has an extremely high luminance in a portion of the frame. As a result, the frame is made dim as a whole by the exposure control referring to the average value. Since, moreover, the exposure control is accomplished in conformity to the average value of the high- and low-luminance portions, there arises a tendency that the high-luminance portion becomes so bright as to blank the image white.
In the case of the exposure control having the central portion or regions weighted, on the other hand, that tendency is considerably removed, if the portion having an extremely high luminance exists outside of the highly weighted portion, but cannot be solved if it exists inside.
In Japanese Patent Laid-Open No. 60-178779, the exposure conditions are determined by extracting the abnormal level only, so that the image pickup conditions have to be changed in various manners to take a long time for the determination. In case the frame is divided into a dark portion, a bright portion (i.e., a main object) and an extremely bright portion (into which the light source is included), as disclosed in Japanese Patent Laid-Open No. 60-246186, for example, the main object becomes dark for the photographing at the white level because the exposure is adjusted to the extremely bright portion. For the photographing at the black level, on the contrary, the main object is blanked white because the exposure is adjusted to the dark portion. In case the image is spotted extremely bright or dark even if it is synthesized and reproduced, the main object becomes blacky or whitish so that an excellent image is not obtained. In this system, the iris is moved while checking the peak values of the white and black levels for the photographing conforming to the individual levels until the photographing is accomplished at the proper level. Therefore, it takes a long time to take two shots, and the exposure timing of this photographing is highly deviated.